Drawing equivalent to practicing scales

I'm always trying to improve my fundamental drawing abilities and while I know that the standard advice is to practice sketching every day, I was wondering if anyone does more fundamental exercises on a regular basis to improve or keep their draftsmanship abilities honed.

I play mandolin and to me sketching feels like the equivalent of playing random songs which is certainly helpful but as a musician, if I want to improve my skills more quickly I do scales or finger exercises as well. Are there daily exercises you do to help improve your motor skills, your ability to get accurate proportions etc.? I guess that's two questions -- are there exercises and do you as an illustrator regularly do them?

Part of the reason I ask this is because I do sketch every day but sometimes I feel like I'm just reinforcing my mistakes. How can I improve my practice time to make it more effective?

I don´t do as much as I should right now, but some things I used to do regularly: timed gesture drawing (there’s a website that gives you timed reference, here), non-timed gesture drawing (tons of reference books you can use, one of my favorites is “Colossal Collection of Action Poses” by Buddy Scalera), hand drawings and head drawing (collections for reference can be found online or in print. For example “Facial expressions from babies to teens” is an excellent one for children illustrators), copying character sheets from animation films (also easy to find online), animal drawing (several per animal type in different poses).
And whenever your mind wanders, you can always draw pages of ellipses

I started regularly doing line-quality exercises in December and my linework's improved leaps and bounds. It's what I do before anything else. Go watch the how to draw anything videos. They've got awesome exercises.

But then again it really depends on what you want to get better at. If you want ultra-smooth inked lines then gesture drawing may not help much. If you want awesome true-to-life proportions, then drawing straight lines won't do much for you either.

My morning routine (though I'm sure there's better ways of doing this) looks like one page of linework, one page of basic shapes in perspective (so cubes, spheres, cylinders, etc) then I go an art prompt generator and I sketch one object, one creature and one character. When I do the art prompts I try to really keep what I'm trying to practice in mind.

@art-of-b This kind of routine is what I'd like to set up for myself. You're right that it partly depends on where I want to improve but it seems like there must be some things that are fundamental to all drawing, and I'm thinking that line work and shapes would fall into that category. Thanks.

I finally got a chance to look at your link to the timed references for gesture drawing and think it will be very useful. Thanks for this and all of your help -- you always have the best suggestions and advice!